i would bet on a seaping head bolt. if memory serves there are a couple head bolts that go thru water passages. if not sealed properly will let water get into the valve cover then down into the block. i wouldnt run it without fresh oil, and a good clean up as you may damage the lower end if any of that coolant got to the pan. as jimmie said, i would try letting it run and get nice and warmed up and see if you can track where its coming from before disassembly. one thing i would do as well would be to put a more coolant strong mixture into the rad before trying to track it down. make sure you got a nice bright green to look for instead of a pale green from having the proper mix.